Alignment target to be measured with multiple polarization states

ABSTRACT

An alignment target includes periodic patterns on two elements. The periodic patterns are aligned when the two elements are properly aligned. By measuring the two periodic patterns at multiple polarization states and comparing the resulting intensities of the polarization states, it can be determined if the two elements are aligned. A reference measurement location may be used that includes third periodic pattern on the first element and a fourth periodic pattern on the second element, which have a designed in offset, i.e., an offset when there is an offset of a known magnitude when the first and second element are properly aligned. The reference measurement location is measured at two polarization states. The difference in the intensities of the polarization states at reference measurement location and is used to determine the amount of the alignment error.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to alignment metrology, and in particularto an alignment target and method of use.

2. Discussion of the Related Art

Semiconductor processing for forming integrated circuits requires aseries of processing steps. These processing steps include thedeposition and patterning of material layers such as insulating layers,polysilicon layers, and metal layers. The material layers are typicallypatterned using a photoresist layer that is patterned over the materiallayer using a photomask or reticle. It is important that one layer isaligned with another during processing.

Typically, the photomask has alignment targets or keys that are alignedto fiduciary marks formed in the previous layer on the substrate.However, as the integrated circuit feature sizes continue to decrease toprovide increasing circuit density, it becomes increasingly difficult tomeasure the alignment accuracy of one masking level to the previouslevel. This overlay metrology problem becomes particularly difficult atsubmicrometer feature sizes where overlay alignment tolerances arereduced to provide reliable semiconductor devices.

FIGS. 1A and 1B show conventional overlay targets used with conventionalimaging metrology methods. FIG. 1A shows a typical Box-in-Box overlaytarget 2. Target 2 is formed by producing an etched box 4 in a materiallayer 6 on a substrate. A corresponding smaller box 8 on the photomaskor reticle is aligned to the larger box 4 so that the centers of thelarge and small boxes are aligned.

FIG. 1B shows a Bar-in-Bar overlay target 12, which is similar to target2 shown in FIG. 1A. Target 12 is produced by etching bars 14 in amaterial layer 16 on a substrate. The bars 18 on the photomask arealigned to the overlay target alignment bars 14.

After the smaller box 8 or bars 18 are developed, i.e., exposed andetched, the overlay target is imaged to determine whether the photomaskor reticle was properly aligned with the underlying layer.Conventionally, high magnification imaging is used to measure overlayalignment. Conventional imaging devices, unfortunately, suffer fromdisadvantages such as sensitivity to vibration and cost. Moreover,conventional imaging devices suffer from a trade-off betweendepth-of-focus and optical resolution. Additionally, edge-detectionalgorithms used to analyze images for the purpose of extracting overlayerror are inaccurate when the imaged target is inherently low-contrastor when the target suffers from asymmetries due to wafer processing.

During processing, the substrate is moved from one location to the nextso that different areas, e.g., dies, on the substrate can be exposed.The alignment system, e.g., the exposure tool, typically uses analignment target to properly align the substrate during exposure. FIG. 2shows a conventional alignment system 50, which includes a diffractionpattern 52 on the substrate and a second diffraction pattern 54 that isstationary, e.g., is fixed to the lens on the exposure tool. A lightsource 56 produces coherent light that passes through a beam splitter 58and is incident on the diffraction pattern 52 after passing through alens 60. The light is diffracted by diffraction pattern 52 and passesthrough lens 60 back to beam splitter 58. The coherent light beam fromsource 56 is also reflected off beam splitter 58, passes through lens 62and is incident on diffraction pattern 54. The light diffracted bydiffraction pattern 54 passes back through lens 62 to beam splitter 58.At beam splitter the light diffracted from diffraction patterns 52 and54 is combined and the combined diffraction patterns is received bylight detectors 64.

Alignment system 50 provides an accuracy of approximately 15 nm. Onedisadvantage of alignment system 50 is that coherent light is used.Thus, if the diffraction pattern 52 on the sample absorbs the particularfrequency used, alignment system 50 cannot provide an accuratemeasurement. While multiple coherent light sources may be used to avoidthis disadvantage, the use of multiple light sources adds complexity andcost.

Thus, there is a need in the semiconductor industry for an improvedalignment target for metrology and alignment system.

SUMMARY

An alignment target in accordance with the present invention is used todetermine if two elements are in alignment or the amount of thealignment error between the two elements. The alignment target includesperiodic patterns on the two elements. The periodic patterns, which maybe, e.g., diffractions gratings, are aligned when the two elements arein alignment. The alignment target is measured by producing light withmultiple polarization states that is incident on the alignment target.The intensities of the polarization states are detected after the lightinteracts with the alignment target. The polarization states can then becompared to determine if the elements are aligned. Additional periodicpatterns that have a designed in offset, i.e., a known offset betweenthe two elements when the elements are aligned, may be used to aid inmeasurement of the alignment error.

In one embodiment, a method includes providing an alignment target on afirst element and a second element, the alignment target having a firstperiodic pattern on the first element and a second periodic pattern onthe second element. The first and second periodic patterns areilluminated with light having at least two polarization states. Thelight may be incident at from one or more directions. The intensities ofthe polarization states of the light after interacting with thealignment target is detected, and the intensities are compared todetermine the alignment of the first element and the second element. Oneor both of the periodic patterns may be diffraction gratings havingperiodicities in one or two directions. The method may include movingone element with respect to the other to minimize the difference betweenthe intensities of the polarization states.

The alignment target may further include a third periodic pattern on thefirst element and a fourth periodic pattern on the second element, thethird periodic pattern and the fourth periodic pattern have a designedin offset of a known magnitude such that when the first element andsecond element are aligned, the third periodic pattern and the fourthperiodic pattern are offset by the known magnitude. The third and fourthperiodic patterns are illuminated with light having at least twopolarization states, and the intensities of the polarization states aredetected and compared. The compared intensities from the third andfourth periodic patterns can be used as a reference for the comparedintensities of the first and second periodic patterns.

Alternatively, a model may be produced of the periodic patterns and thelight with multiple polarization states using e.g., rigorous coupledwave analysis (RCWA). The measured intensities of the periodic patternsare compared with the model to determine the alignment error.

In another embodiment of the present invention, an alignment target formeasuring the alignment between a first element and a second elementincludes a first location having a first periodic pattern on the firstelement and a second periodic pattern on the second element. The secondperiodic pattern is aligned to the first periodic pattern when the firstelement and the second element are properly aligned. The alignmenttarget also includes a second location having a third periodic patternon the first element and a fourth periodic pattern on the secondelement. The fourth periodic pattern has a designed in offset of a knownmagnitude with the third periodic pattern when the first element and thesecond element are properly aligned.

In another embodiment, an apparatus for determining the alignment of afirst element with a second element using an alignment target having afirst periodic pattern on said first element and a second periodicpattern on said second element, includes a radiation source forproducing radiation having at least two polarization states to beincident on the alignment target. The apparatus further includes adetector for detecting the radiation with at least two polarizationstates after it interacts with the alignment target; and a computer anda computer-usable medium having computer-readable program code embodiedtherein for causing the computer to calculate the difference between theintensities of the at least two polarization states to determine if thefirst element and the second element are aligned.

Where the alignment target includes a second measurement location with athird and fourth periodic patterns, the radiation source producesradiation having at least two polarization states to be incident on boththe first and second periodic patterns and the third and fourth periodicpatterns, and the detector detects the radiation with at least twopolarization states after it interacts with both the first and secondperiodic patterns and the third and fourth periodic patterns. Thecomputer-readable program code embodied in the computer-usable mediumcauses the computer to compare the intensities of the polarizationstates from the light after interacting with the third periodic patternand the fourth periodic pattern and using the comparison to determinethe amount of alignment error between the first element and the secondelement.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show conventional overlay targets used with conventionalimaging metrology methods.

FIG. 2 shows a conventional alignment system, which includes adiffraction pattern on the substrate and a second diffraction patternthat is stationary, e.g., is fixed to the lens on the exposure tool.

FIGS. 3A and 3B show a top and cross-sectional views, respectively, ofan alignment target in accordance with an embodiment of the presentinvention.

FIGS. 4 and 5 shows the alignment target in accordance with the presentinvention with an alignment error.

FIG. 6 is a block diagram of an alignment system with which thealignment target may be used.

FIG. 7 shows a perspective view of a substrate and a reticle with a lensand four reference masks disposed between the substrate and reticle.

FIGS. 8A, 8B, 8C show metrology devices that may be used to measure thediffraction from an alignment target in accordance with the presentinvention.

FIGS. 9A, 9B, 9C, 9D, 9E, and 9F show the alignment target beingmeasured from different angles simultaneously.

FIGS. 10 and 11 show a cross-sectional and top view, respectively, of analignment target in accordance with another embodiment of the presentinvention.

FIG. 12 shows top view of an alignment target in accordance with anotherembodiment of the present invention.

FIG. 13 shows measuring non-zero order diffracted light from analignment mark.

DETAILED DESCRIPTION

An alignment target, in accordance with the present invention, can beused to align two elements. For example, the alignment target can beused to align a substrate and/or a reticle with respect to a stationaryelement, e.g., the lens, in an exposure tool. Of course, the alignmenttarget is not limited to use in an exposure tool, but may be used toalign any two elements. Additionally, the alignment target can be usedto measure the amount of alignment error between any two elements, suchas two layers on a substrate or any other elements.

The alignment target of the present invention and method of use issimilar to the alignment target used to assist in the alignment processand to measure alignment error as described in U.S. patent applications“Positioning Two Elements Using an Alignment Target With A Designed InOffset” by Weidong Yang, Roger R. Lowe-Webb, John D. Heaton and GuoguangLi, Ser. No. 10/116,964; “Alignment Target with Designed in Offset” byWeidong Yang, Roger R. Lowe-Webb, and John D. Heaton, Ser. No.10/116,863, now U.S. Pat. No. 6,982,793; and “Encoder with AlignmentTarget”, by Weidong Yang, Roger R. Lowe-Webb, and John D. Heaton, Ser.No. 10/116,855, now U.S. Pat. No. 6,958,819, all of which are filedherewith and have the same assignee as the present application and areincorporated herein by reference.

FIGS. 3A and 3B show a top and cross-sectional views, respectively, ofan alignment target 100 in accordance with an embodiment of the presentinvention. Alignment target 100 includes a periodic pattern 102 on afirst element 103 and another periodic pattern 104 on a second element105. The periodic patterns 102, 104, which may be diffraction gratings,are designed to be in alignment with respect to each other when layers103 and 105 are in alignment. The first element 103 and second element105 may be movable with respect to each other. For example, the firstand second element 103 and 105 may be two stages or a substrate and/orreticle and another object, such as a lens or a reference mask coupledto a lens, in an exposure tool. The elements 103 and 105 may also belayers on a substrate, such as a semiconductor wafer, flat panel displayor any other structure in which accurate alignment of successive layersis desired. The periodic patterns of alignment target 100 are similar tothat described in U.S. Ser. No. 09/960,892 entitled “SpectroscopicallyMeasured Overlay Target”, filed Sep. 20, 2001, which has the sameassignee as the present disclosure and which is incorporated herein inits entirety by reference.

The bottom periodic pattern 102 is produced, for example, by providing alayer of appropriate material, such as a 200 nm layer of polysilicon,followed by a layer of photoresist. The desired image including theperiodic pattern 102 is exposed in the photoresist, which is thendeveloped. The polysilicon is then etched away leaving periodic pattern102.

The top periodic pattern 104 is produced using, e.g., photoresist, in amanner similar to the bottom periodic pattern 102. The top periodicpattern 104 may be separated from the bottom periodic pattern 102 by oneor more intermediate layers. Thus, for example, an 800 nm layer of thephotoresist may be used to produce top periodic pattern 104. The desiredimage including the periodic pattern is exposed in the photoresistlayer. The photoresist may then be developed to produce periodic pattern104, or alternatively, a latent image may be used as periodic pattern104.

It should be understood that the processing steps used to produceperiodic patterns 102 and 104 are exemplary. Different or additionallayers may be included between substrate and the bottom periodic pattern102 or between the bottom periodic pattern 102 and the top periodicpattern 104. In fact, if desired, the top periodic pattern 104 may be onthe same layer as the bottom periodic pattern 102, in which case, topand bottom refers to the order in which the gratings are produced, i.e.,the bottom periodic pattern being first and the top periodic patternbeing second. Thus, the alignment target 100 may be used to ensure thata first pattern produced on a first layer on the substrate is alignedwith a second pattern produced on the same layer on the substrate.Moreover, the alignment target 100 may be used with two elements thatare not connected. Thus, for example, the top periodic pattern 104, andthe bottom periodic pattern 102 may be separated by a small distance,e.g., up to approximately 50 μm or more.

The dimensions of the patterns and the thicknesses of the layers may bealtered. For example, the bottom diffraction grating 102 need not extendto the top of element 105. It should be understood that the alignmenttarget 100 may be produced using various materials and the dimensionsoptimized for the materials used. Thus, the dimensions of the alignmenttarget 100 may be altered to maximize sensitivity based on the types ofmaterials used.

To determine if the layers 103, 105 are in alignment, a radiation source120, such as a broadband light source, produces radiation 121 that isincident on alignment target 100. Source 120 produces light that has anon-normal angle of incidence and has an azimuthal angle that isnon-parallel with the direction of periodicity of the periodic patternsin alignment target 100, if the pattern has only one periodicitydirection.

The radiation source 120 produces radiation 121 that has a plurality ofpolarization states, e.g., two polarization states, as illustrated inFIG. 3B. After the radiation interacts with alignment target 100, adetector 122 detects the radiation. The difference in intensity of thepolarization states from alignment target 100 vary proportionally withthe alignment error. When the elements 103 and 105 are in alignment,periodic patterns 102 and 104 will be in alignment. Consequently, thepolarization states in the detected radiation will have equal intensity.However, if there is an alignment error between elements 103 and 105,periodic patterns 102 and 104 will be out of alignment, as illustratedin FIGS. 4 and 5. With the periodic patterns 102 and 104 out ofalignment, the intensity of the detected polarization states will beunequal. Thus, by comparing the intensities of the detected polarizationstates from alignment target 100, it is possible to determine ifelements 103 and 105 are in alignment.

The ability to determine if elements 103 and 105 are in alignment isparticularly useful in an alignment system. Thus, for example, thepresent invention may be used to ensure substrate to reticleregistration when the substrate is on the exposure tool duringprocessing. The alignment target may be used to assist in the precisealignment of separate elements in any alignment system and is notlimited to an exposure tool.

It should be understood that the present invention may be used in bothreflection and transmission modes.

The present invention may be used to measure the alignment error down toa fraction of a nanometer, while the current industry standard isapproximately 15 nm. Thus, the present invention provides a largeimprovement compared to current technology.

FIG. 6 is a block diagram of an alignment system with which thealignment target 100 may be used. The alignment system is an exposuretool 200 includes X and Y substrate stages 202 and 204 that hold thesubstrate 201. The exposure tool 200 also includes X and Y reticlestages 206 and 208 that hold the reticle 210. Exposure tool 200 mayinclude two sets of stages, one set for large motions and another setfor fine motions. For sake of simplicity, X and Y stages 202, 204, 206,and 208 may be used for both large motion and fine motion.

A control system 212 controls the motion of the stages. A lens 214 orother suitable optics is positioned between the substrate 201 and thereticle 210 and is used to focus light from light source 216 that istransmitted through reticle 210 onto substrate 201. The operation andcontrol of exposure tools is well known in the art.

A reference mask 218 extends from the lens 214 by way of a low thermalexpansion arm 219. The distance between the reference mask 218 and thesubstrate 201 should be small, e.g., between 1 and 10 μm. Spectrometers220 are positioned above reference masks 218. As shown in FIG. 6, aplurality of reference masks 218 may be used, each having an associatedspectrometer 220.

FIG. 7 shows a perspective view of substrate 201 and reticle 210 withlens 214 and four reference masks 218 disposed between the substrate 201and reticle 210. The spectrometers 220 are not shown in FIG. 16. As canbe seen in FIG. 7, a number of separate alignment targets 100 are used,where the top periodic pattern 104 is on the reference masks 218 andbottom periodic pattern 102 is on the substrate 201.

Referring back to FIG. 6, the spectrometers 220 may be coupled to thelens 214 as shown in FIG. 6, or may be connected to another stationaryobject. The spectrometers 220 communicate with the control system 212.The control system 212 includes, e.g., a computer-usable medium havingcomputer-readable program code embodied therein for causing the controlsystem to calculate the difference between the diffracted light from thetwo locations and using the difference to determine if the first elementand the second element are aligned. The control system 212 is coupled tothe stage to adjust the location of the substrate 201 in response to thesignals provided by spectrometer 220 until the starting position of thesubstrate 201 is precisely aligned with the lens 214. Once the substrate201 is aligned, the control system 212 can move the stages to performthe desired exposure operation.

Alignment target 100 may be used to measure the amount of alignmenterror, e.g., using several types of metrology devices, e.g., such asthat shown in FIGS. 8A, 8B, and 8C. FIG. 8A, for example, shows a blockdiagram of a normal incidence polarized reflectance spectrometer.Spectrometer 300 is discussed in detail in the U.S. patent applicationentitled “Apparatus and Method for the Measurement of DiffractingStructures,” filed Sep. 25, 2000, having Ser. No. 09/670,000, and theU.S. patent application entitled “Measurement Of Diffracting StructuresUsing One-Half Of The Non-Zero Diffracted Orders” filed Apr. 27, 2000,having Ser. No. 09/844,559, both of which have the same assignee as thepresent disclosure and are incorporated herein by reference.Spectrometer 300 may use rigorous coupled wave analysis (RCWA) asdescribed in Ser. No. 09/670,000, or folded rigorous coupled waveanalysis as described in Ser. No. 09/844,559 to measure the alignmenterror from alignment target 100. In an alignment system, such as thatshown in FIG. 6, however, a precise measurement is not necessary. Only adetermination of whether the polarization states have the same intensityneeds to be made in order to determine if the periodic patterns 102 and104 are in alignment.

As shown in FIG. 8A, spectrometer 320 includes a polychromatic lightsource 322 that generates a light beam that is partially reflected bybeam splitter 324 along the optical axis 323. The light beam is directedtowards a sample 326 having an alignment target, such as alignmenttarget 100, to be measured. Sample 326 may be, e.g., a semiconductorwafer or flat panel display or any other substrate, and is supported bya stage 328, which may be a polar coordinate, i.e., R-θ, stage or an x-ytranslation stage. Spectrometer 320 includes a rotatable polarizer 330and a lens 332 (or series of lenses) to polarize and focus the lightbeam onto the sample 326 at normal incidence. The beam is reflected offsample 326 and the reflected light is transmitted through lens 332 andpolarizer 330. A portion of the reflected light is transmitted throughbeam splitter 324 and is received by a spectrophotometer 334.Spectrophotometer 334 is coupled to processor 336, which analyzes thedata provided by spectrophotometer 334. Processor 336 is e.g., acomputer with a computer and a computer-usable medium havingcomputer-readable program code embodied therein for causing the computerto determine the overlay error based on the light that is diffractedfrom the alignment target. Thus, the computer-readable program codecauses the computer to calculate the difference between the intensitiesof the polarization states from the alignment target and use thedifference to determine if the alignment target is aligned. One ofordinary skill in the art can program code necessary to determinealignment in accordance with the present invention in view of thepresent disclosure. For more information on the general operation of anormal incidence polarized reflectance spectrometer, the reader isreferred to Ser. Nos. 09/670,000 and 09/844,559, which are incorporatedherein by reference.

If desired, other measurement devices may be used to measure alignmenttarget 100 including ellipsometry and scatterometry. FIGS. 8B and 8Cshow block diagrams of a spectroscopic ellipsometer 340 andspectroscopic scatterometer 345. Ellipsometer 340 includes a broadbandradiation source 341 and a spectrophotometer 342, which is connected toa processor 343. Spectroscopic scatterometer 345 includes a broadbandradiation source 346 and a spectrophotometer 347, which is connected toa processor 348. As indicated by the arrows in FIG. 8C, one or both ofbroadband radiation source 346 and spectrophotometer 347 are adjustableto alter the angle of incidence. The operation of an ellipsometer 340and spectroscopic scatterometer 345 is well known to those skilled inthe art. Processor 343 and 348 may be similar to processor 336 shown inFIG. 8A.

It should also be understood, that the present invention may use asingle wavelength, a plurality of discrete wavelengths, or a continuumof wavelengths.

If desired, multiple light sources and detectors may be usedsimultaneously. FIG. 9A for example, shows alignment target 100 beingmeasured from different angles simultaneously with a plurality ofradiation beams, 121 a, 121 b, 121 c. Because of the symmetricalproperties of the periodic patterns in alignment target 100, themeasurement may be made at multiple symmetrical angles.

FIGS. 9B, 9C and 9D illustrate the symmetries of alignment target 100.As can be seen in FIGS. 9B, 9C, and 9D, alignment target 100 has mirrorsymmetry along the x-z plan, the y-z plan and 180 degree rotationsymmetry around the z axis. The y-z plan mirror symmetry and z axisrotation symmetry are broken once the alignment pattern symmetry isbroken, i.e., when there is an alignment error.

As illustrated in FIGS. 9B, 9C, and 9D, incident light has polarizationcomponents along s and p, with equivalent phase shifts shown in eachfigure. The light that is detected has polarization states s′ and p′where there is also equivalent phase shifts shown in each figure. Itshould be understood that the phase shift in the incident light and thedetected light need not be the same. Moreover, the polarization statesneed not be linear. Because of symmetry, the measurements in FIGS. 9Cand 9D are identical and are also identical to the measurement in FIG.9B, but only when the alignment patter is symmetrical. This differencecan be used for alignment as well as shift measurement.

A special case is when the incident light comes in from the y directionas shown in FIGS. 9E and 9F. In this case, the two measurements, a andb, follow the same light path, but with both the incidence and detectingpolarizations mirroring each other in the y-z plan. As can be seen inFIGS. 9E and 9F, the polarization angels used in the incident light neednot be used in the detection.

It should be understood that multiple polarization states may be used todetermine when the symmetry in the alignment target is broken. Forexample, in one embodiment, white light may be used to determine whenthe symmetry is broke.

In a special case, the polarization states of the two beams may overlapin either the incident beam or in the detection of the resulting beam.

The present invention may be used to not only determine if the elementsare aligned, but to measure the alignment error if any. The alignmenterror may be measured using alignment target 100, for example, using amodeling technique, such as RCWA. Alternatively, a reference pattern maybe used.

To measure the alignment error using a modeling technique, a model ofthe alignment target and the incident and diffracted light. The modeledlight is compared to the measured light to determine if there is anacceptable fit, i.e., the difference between the modeled light andmeasured light is within a specified tolerance. If an acceptable fit isnot found, the alignment target and incident and diffracted light areremodeled and compared to the measured light. Once an acceptable fitbetween the measured and modeled light is found, it is known that themodeled alignment target accurately describes the actual alignmenttarget. If desired, the difference in the spectra from the twopolarization states may be determined before or after the comparisonwith the modeled light. Of course, if the difference in measured spectrais used, the model light must be the difference in the modeled spectra.Moreover, a number of models of the alignment target, including themodeled light, may be generated a priori and stored in a library.

FIG. 10 shows a cross-sectional view of an alignment target 400 inaccordance with another embodiment of the present invention. Alignmenttarget 400 includes two measurement locations, referred to as overlaypatterns 402 and 404. Each of the overlay patterns 402, 404 includes aperiodic pattern (402 a, 404 a) on a first element 403 and a periodicpattern (402 b, 404 b) on a second element 405. The overlay pattern 402has no designed in offset, i.e., periodic patterns 402 a and 402 b arealigned when elements 403 and 405 are in proper alignment. Overlaypattern 404, however, has a designed in offset between the bottomperiodic pattern 404 a and 404 b of a magnitude D.

It should be understood that periodic patterns 402 b and 404 b orperiodic patterns 402 a and 404 a may be a parts of the same continuousperiodic pattern. Because the measurement is made at differentlocations, however, it is unimportant if the patterns are connected ornot.

FIG. 11 shows a top view of alignment target 400. The periodic patternsin overlay patterns 402 and 404 may be oriented at any non-perpendicularangle with respect to each other.

In operation, overlay pattern 404 is used as a reference pattern foralignment target 400. Each measurement location, i.e., overlay patterns402 and 404, of alignment target 400 is measured at a plurality of,e.g., two, polarization states. When elements 403 and 405 are properlyaligned, the intensities of the polarization states from overlay pattern402 will be equal, but the intensities of the polarization states fromoverlay pattern 404 will be unequal. When there is an alignment errorbetween the elements 403 and 405, the intensities of the polarizationstates from overlay pattern 402 will be unequal. Because the differencein the intensities of the polarization states varies proportionally withthe alignment error, the difference in intensities of the polarizationstates from overlay pattern 404 may be used as a reference measurement.

In general, the alignment error e is determined by:

$\begin{matrix}{e = {\frac{\varphi_{1}}{\varphi_{2} - \varphi_{1}}*D}} & {{eq}.\mspace{14mu} 1}\end{matrix}$wherein φ₁ is the differential spectra at the target location, i.e.,overlay pattern 402, φ₂ is the differential spectra at the referencelocation, i.e., overlay pattern 404, and D is the designed in offset atthe reference location. By optimizing equation 1, the alignment error efor the entire spectrum, equation 1 the alignment error is determinedas:

$\begin{matrix}{e = {\frac{\sum\limits_{i}{\varphi_{1,i}*( {\varphi_{2} - \varphi_{1}} )_{i}}}{\sum\limits_{i}( {\varphi_{2} - \varphi_{1}} )_{i}^{2}}*D}} & {{eq}.\mspace{14mu} 2}\end{matrix}$where i is the wavelength number in the spectrum.

In another embodiment, the reference location is produced by moving thesecond element 405 with respect to the first element 403 by a distance Dand measuring the pattern in the new position. In this embodiment, thesecond overlay pattern 404 is not necessary.

When the relationship between the differential spectra and the alignmenterror is assumed to be a polynomial, the higher orders can be treated byusing additional reference patterns. The use of additional referencepatterns and a polynomial equation to solve for the alignment error isdiscussed in more detail in U.S. patent application entitled “AlignmentTarget with Designed in Offset” by Weidong Yang, Roger R. Lowe-Webb,John D. Heaton, and Guoguang Li, which is incorporated herein.

FIG. 12 shows top view of an alignment target 500 in accordance withanother embodiment of the present invention. Alignment target 500 is abi-grating includes two periodic patterns, a bottom periodic pattern502, shown as a solid squares, and a top periodic pattern 504, shown asempty squares, that have periodicities in two directions. The twoperiodicities need not be perpendicular and the shapes forming theperiodic patterns need not be square. Because the alignment target 500has two periodicities, it can be used to determine alignment in twodirections, shown as the X and Y direction in FIG. 12. Alignment target500 is measured from two separate directions by beam 506, which issensitive to alignment error in the Y direction and beam 508, which issensitive to alignment error in the X direction.

Moreover, it should be understood that one or both of the periodicpatterns, e.g., in alignment target 100 FIGS. 3A, 3B can be a bi-gratingif desired.

Further, if desired, the incident light and the detected light need notshare the same azimuthal angle. Thus, as shown in FIG. 13, an alignmentmark 600, which is similar to alignment mark 100, may be measured by alight source 620 that produces an incident light beam 621 and a detector624 detects non-zero order diffracted light 622.

In general, the cross-reflection symmetry may be used for the errormeasurement and alignment control. The reflection of the light from asurface can be expressed as:E _(out) =R·E _(in)  eq. 3where

$\begin{matrix}{E = \begin{pmatrix}E_{s} \\E_{p}\end{pmatrix}} & {{eq}.\mspace{14mu} 4} \\{and} & \; \\{R = \begin{bmatrix}r_{pp} & r_{sp} \\r_{ps} & r_{ss}\end{bmatrix}} & {{eq}.\mspace{14mu} 5}\end{matrix}$r_(sp) and r_(ps) are defined as the cross-reflection coefficients. Forsymmetric grating, the 0^(th) order cross-reflection coefficients areknown to be identical but with a sign change in conical mount. Withsymmetry broken, they no longer have identical magnitude. This propertycan be exploited for alignment control and overlay error measurement.

Let s as x axis, and p as y axis. In conical incidence, if the gratingis symmetrical, 0^(th) orders are antisymmetrical, r_(sp)=−r_(ps)≠0,while the higher orders are symmetrical, r_(sp)=r_(ps). Fortransmission, this relationship is reversed. The 0^(th) orders aresymmetrical, while higher orders are antisymmetrical. Assume incidencelight has polarization along angle θ₁, with phase different φ₁ between sand p polarizations. Also assume that the 0^(th) order refection isdetected at polarization along angle θ₂, with phase different φ₂ betweens and p polarizations. Incidence light can be described as:

$\begin{matrix}{E_{in} = {\begin{pmatrix}E_{s} \\E_{p}\end{pmatrix} = \begin{pmatrix}{\sin( \theta_{1} )} \\{{\cos( \theta_{1} )}{\mathbb{e}}^{{\mathbb{i}}\;\phi_{1}}}\end{pmatrix}}} & {{eq}.\mspace{14mu} 6}\end{matrix}$0^(th) order reflection:

$\begin{matrix}{E_{out} = {{R \cdot E_{in}} = {{\begin{bmatrix}r_{pp} & r_{sp} \\r_{ps} & r_{ss}\end{bmatrix} \cdot \begin{pmatrix}{\sin( \theta_{1} )} \\{{\cos( \theta_{1} )}{\mathbb{e}}^{{\mathbb{i}\phi}_{1}}}\end{pmatrix}} = \begin{pmatrix}{{r_{pp}{\sin( \theta_{1} )}} + {r_{sp}{\cos( \theta_{1} )}{\mathbb{e}}^{{\mathbb{i}\phi}_{1}}}} \\{{r_{ps}{\sin( \theta_{1} )}} + {r_{ss}{\cos( \theta_{1} )}{\mathbb{e}}^{{\mathbb{i}\phi}_{1}}}}\end{pmatrix}}}} & {{eq}.\mspace{14mu} 7}\end{matrix}$The measured electric field is:

$\begin{matrix}{E_{measure} = {{( {{\sin( \theta_{2} )}{\cos( \theta_{2} )}{\mathbb{e}}^{{\mathbb{i}\phi}_{2}}} ) \cdot E_{out}} = {{( {{\sin( \theta_{2} )}{\cos( \theta_{2} )}{\mathbb{e}}^{\mathbb{i}\phi 2}} ) \cdot \begin{pmatrix}{{r_{pp}{\sin( \theta_{1} )}} + {r_{sp}{\cos( \theta_{1} )}{\mathbb{e}}^{{\mathbb{i}\phi}_{1}}}} \\{{r_{ps}{\sin( \theta_{1} )}} + {r_{ss}{\cos( \theta_{1} )}{\mathbb{e}}^{{\mathbb{i}\phi}_{1}}}}\end{pmatrix}} = {{r_{pp}{\sin( \theta_{1} )}{\sin( \theta_{2} )}} + {r_{ss}{\cos( \theta_{1} )}{\cos( \theta_{2} )}{\mathbb{e}}^{{\mathbb{i}}{({\phi_{1} + \phi_{2}})}}} + {r_{sp}{\sin( \theta_{2} )}{\cos( \theta_{1} )}{\mathbb{e}}^{{\mathbb{i}\phi}_{1}}} + {r_{sp}{\cos( \theta_{2} )}{\sin( \theta_{1} )}{\mathbb{e}}^{{\mathbb{i}\phi}_{2}}}}}}} & {{eq}.\mspace{14mu} 8}\end{matrix}$

A second measurement is made along the same light path. Assume theincidence light has polarization along angle θ₂, with phase differentφ₁′ between s and p polarizations. Also assume that the 0^(th) orderrefection is detected at polarization along angle θ₁, with phasedifferent φ₂′ between s and p polarizations. The measured electric fieldis:

$\begin{matrix}{E_{measure} = {{( {{\sin( \theta_{1} )}{\cos( \theta_{1} )}{\mathbb{e}}^{{\mathbb{i}\phi}_{2}^{\prime}}} ) \cdot \begin{bmatrix}r_{pp} & r_{sp} \\r_{ps} & r_{ss}\end{bmatrix} \cdot \begin{pmatrix}{\sin( \theta_{2} )} \\{{\cos( \theta_{2} )}{\mathbb{e}}^{{\mathbb{i}\phi}_{1}^{\prime}}}\end{pmatrix}} = {{r_{pp}{\sin( \theta_{1} )}{\sin( \theta_{2} )}} + {r_{ss}{\cos( \theta_{1} )}{\cos( \theta_{2} )}{\mathbb{e}}^{{\mathbb{i}}{({\phi_{1}^{\prime} + \phi_{2}^{\prime}})}}} + {r_{sp}{\sin( \theta_{1} )}{\cos( \theta_{2} )}{\mathbb{e}}^{{\mathbb{i}\phi}_{1}^{\prime}}} + {r_{sp}{\cos( \theta_{1} )}{\sin( \theta_{2} )}{\mathbb{e}}^{{\mathbb{i}\phi}_{2}^{\prime}}}}}} & {{eq}.\mspace{14mu} 9}\end{matrix}$

To obtain symmetrical measurements for symmetrical grating usingsymmetrical reflection or transmission orders, i.e. r_(sp)=r_(ps), ort_(sp)=t_(ps), the following condition has to be satisfied:e ^(i(φ) ¹ ^(+φ) ² ⁾ =e ^(i(φ) ¹ ^(′+φ) ² ^(′))e^(iφ) ¹ =e^(iφ) ¹ ^(′)e^(iφ) ² =e^(iφ) ² ^(′)  eq. 10which can be simplified as:φ₁=φ₁′φ₂=φ₂′  eq. 11

To obtain symmetrical measurements for symmetrical grating usingantisymmetrical reflection or transmission orders, i.e. r_(sp)=−r_(ps),or t_(sp)=−t_(ps), the following condition has to be satisfied:e ^(i(φ) ¹ ^(+φ) ² ⁾ =e ^(i(φ) ¹ ^(′+φ) ² ^(′))e ^(iφ) ¹ =−e ^(iφ) ¹ ^(′)e ^(iφ) ² =−e ^(iφ) ² ^(′)  eq. 12

Some special cases for antisymmetrical orders are as follows:

-   -   1) First and second incidence lights have polarization which is        mirror symmetrical to s direction linear or non-linear, as does        the first and second detection polarization, linear or        non-linear.    -   2) First and second incidence lights have polarization which is        mirror symmetrical top direction linear or non-linear, as does        the first and second detection polarization, linear or        non-linear.    -   3) One of the polarization directions is s or p.    -   4) One of the polarization directions is s, the other is p.        Effectively, one measurement is done with incidence polarization        of p and detection polarization of s, the other is reversed.

It should be understood that the incidence paths for the twomeasurements do not need to be along the same path. The incidence pathscould be mirror symmetry pair of x-z plan, where x is the grating vectordirection.

Although the invention has been described with reference to particularembodiments, the description is only an example of the invention'sapplication and should not be taken as a limitation. Various otheradaptations and combinations of features of the embodiments disclosedare within the scope of the invention as defined by the followingclaims.

1. An alignment target for measuring the alignment between a firstelement and a second element, said alignment target comprising: a firstlocation comprising: a first periodic pattern on said first element; asecond periodic pattern on said second element; wherein said firstperiodic pattern and said second periodic pattern have the same pitchand wherein said second periodic pattern is aligned to said firstperiodic pattern when said first element and said second element areproperly aligned; and a second location comprising: a third periodicpattern on said first element; a fourth periodic pattern on said secondelement; wherein said third periodic pattern and said fourth periodicpattern have the same pitch and wherein said fourth periodic pattern hasa designed in offset of a known magnitude with said third periodicpattern when said first element and said second element are properlyaligned.
 2. The alignment target of claim 1, wherein said first periodicpattern, said second periodic pattern, said third periodic pattern andsaid fourth periodic pattern are diffraction gratings with linesextending in a direction perpendicular to a first direction.
 3. Thealignment target of claim 1, wherein at least one of said first periodicpattern, said second periodic pattern, said third periodic pattern, andsaid fourth periodic pattern has periodicities in two directions.
 4. Thealignment target of claim 3, wherein said designed in offset is alongone direction and said third periodic pattern has a second designed inoffset with a second known magnitude in a second direction with saidfourth periodic pattern when said first element and said second elementare properly aligned.
 5. The alignment target of claim 1, wherein saidfirst element is a first layer on a substrate and said second element isa second layer on said substrate.
 6. The alignment target of claim 1,wherein said first element is a first pattern produced on a first layeron substrate and said second element is a second pattern produced onsaid first layer on said substrate.
 7. The alignment target of claim 6,wherein said first pattern is formed from a first material and saidsecond pattern is formed from a second material that is different thansaid first material.
 8. The alignment target of claim 7, wherein saidsecond material is photoresist.
 9. An apparatus for determining thealignment of a first element with a second element using an alignmenttarget having a first periodic pattern on said first element and asecond periodic pattern on said second element, said apparatuscomprising: a radiation source for producing radiation having at leasttwo polarization states to be incident on said alignment target; adetector for detecting the radiation with at least two polarizationstates after it interacts with said alignment target; and means forcalculating the difference between the intensities of the at least twopolarization states to determine if the first element and the secondelement are aligned.
 10. The apparatus of claim 9, wherein said firstelement is a first layer on a substrate and said second element is asecond layer on said substrate.
 11. The apparatus of claim 9, whereinsaid first element is a first pattern produced on a first layer on asubstrate and said second element is a second pattern produced on saidfirst layer on said substrate.
 12. The apparatus of claim 9, furthercomprising a stage controlled by said computer, said stage moving saidfirst element with respect to said second element to align said firstelement and said second element.
 13. The apparatus of claim 9, whereinsaid means for calculating comprises a computer and a computer-usablemedium having computer-readable program code embodied therein forcausing said computer to calculate the difference between theintensities of the at least two polarization states to determine if thefirst element and the second element are aligned.
 14. An apparatus fordetermining the alignment of a first element with a second element usingan alignment target having a first periodic pattern on said firstelement and a second periodic pattern on said second element, saidapparatus comprising: a radiation source for producing radiation havingat least two polarization states to be incident on said alignmenttarget; a detector for detecting the radiation with at least twopolarization states after it interacts with said alignment target; and acomputer and a computer-usable medium having computer-readable programcode embodied therein for causing said computer to calculate thedifference between the intensities of the at least two polarizationstates to determine if the first element and the second element arealigned, wherein said computer-readable program code embodied in saidcomputer-usable medium causes said computer to perform the steps of:comparing the calculated difference with a library of differences ofdetected radiation to determine the alignment error.
 15. An apparatusfor determining the alignment of a first element with a second elementusing an alignment target having a first periodic pattern on said firstelement and a second periodic pattern on said second element, saidapparatus comprising: a radiation source for producing radiation havingat least two polarization states to be incident on said alignmenttarget; a detector for detecting the radiation with at least twopolarization states after it interacts with said alignment target; and acomputer and a computer-usable medium having computer-readable programcode embodied therein for causing said computer to calculate thedifference between the intensities of the at least two polarizationstates to determine if the first element and the second element arealigned, wherein: said alignment target includes a third periodicpattern on said first element and a fourth periodic pattern on saidsecond element, said third periodic pattern and said fourth periodicpattern have a designed in offset of a known magnitude such that whensaid first element and second element are aligned, said third periodicpattern and said fourth periodic pattern are offset by said knownmagnitude; said radiation source produces radiation having at least twopolarization states to be incident on both said first and secondperiodic patterns and said third and fourth periodic patterns; saiddetector detects the radiation with at least two polarization statesafter it interacts with both said first and second periodic patterns andsaid third and fourth periodic patterns; and said computer-readableprogram code embodied in said computer-usable medium causes saidcomputer to perform the step of: comparing said intensities of saidpolarization states from said light after interacting with said thirdperiodic pattern and said fourth periodic pattern; and calculating theratio of the comparison of said intensities of said polarization statesfrom said light after interacting with said third periodic pattern andsaid fourth periodic pattern and the comparison of said intensities ofsaid polarization states from said light after interacting with saidfirst periodic pattern and said second periodic pattern to determine theamount of alignment error between said first element and said secondelement.